In certain manufacturing processes, a heat exchanger is used to transfer heat between a first fluid stream used in the process and a second fluid stream. One common type of such a heat exchanger is called a “shell and tube” heat exchanger. A shell and tube heat exchanger (STHE) has a cylindrical casing having first and second ends. An axis extends between the first and second ends of the casing. A plurality of tubes within the casing extends nearly the entire length thereof. A large industrial STHE may have literally thousands of tubes each displaced radially within the casing from those adjacent to allow fluid to flow radially between individual tubes as well as axially along the axis of the casing.
One type of STHE includes a first end plate having a first inlet port therein. The first end plate is sealed to the first end of the casing, say by a continuous weld between the edge of the end plate and the edge of the casing's first end. A second end plate having a first outlet port seals the second end of the casing in a similar way.
A first tubesheet spaced from the first end plate forms with the first end plate and the adjacent casing wall, a first plenum space, or more briefly, first plenum. The adjacent ends of the tubes all penetrate the tubesheet and open into the first plenum. The exterior walls of the tubes are sealed to the first tubesheet to isolate the first plenum from the remaining space within the casing. The first plenum receives the first fluid stream at the first inlet port and distributes the first fluid stream to the open ends of the individual tubes within the first plenum. The first inlet port can also be placed in the casing between the first end plate and the first tubesheet.
A second tubesheet spaced from the second end plate forms with the second end plate and the adjacent casing wall, a second plenum space (second plenum). The adjacent ends of the tubes all penetrate the second tubesheet. The exterior walls of the tubes are sealed to the second tubesheet to isolate the second plenum from the space between the tubesheets. The second plenum directs the first fluid stream flowing from the individual tubes to the first outlet port. The first outlet port can also be placed in the casing between the second end plate and the second tubesheet.
A second inlet port is mounted in the casing between the tubesheets near either the first or second tubesheet. A second outlet port is mounted in the casing between the tubesheets near the other of the tubesheets. The second inlet port receives the second fluid stream which flows across the tubes to the second outlet port.
Since the tubes are often long with relatively thin walls, lateral support is often necessary to prevent them from bending or vibrating during operation of the STHE. This problem is often addressed by a plurality of baffles formed from a metal sheet and located between the tubesheets. Each baffle extends substantially perpendicular to the axis. Each baffle has preformed holes through which the tubes pass to provide this lateral support for the tubes. The baffles are held in axial position by welding to support rods axially extending through the casing, or even to the casing itself. A typical STHE may have anywhere from one to a dozen or more baffles depending on the length of the STHE. If the STHE is relatively short, no baffles may be necessary. In a long STHE with thin-walled tubes, baffles may be placed as close to each other as six inches.
A portion of a baffle's periphery is often in close proximity to the inside of the casing. In other cases, the baffles may be centrally located with arms or spokes extending radially to the casing, or may be held in place with support rods.
Each baffle is usually pierced by at least half of the tubes, but substantially less than all of them. Adjacent baffles are often arranged to project from opposite sides of the casing, so that fluids passing from the second inlet port to the second outlet port have a somewhat serpentine path through the interior of the casing. The baffles also assure that flow of the second fluid stream is directed toward at least part of the length of most of the tubes.
Some STHEs may have series-connected tubes so that flow entering a particular tube traverses the length of the casing two, three, or more times. Where the flow traverses the casing an even number of times, both plenums will typically be on the same end of the casing. All of these variations are well known to those familiar with the technology. The invention to be described can be easily adapted for any of the designs.
This conventional design of a STHE has some flaws. A large pressure drop in the second fluid stream is sometimes present between the second inlet port and the second outlet port. A substantial portion of this pressure drop results from the serpentine path the fluid must take around the baffles. This large pressure drop requires a larger capacity pump or blower. It would be desirable in some circumstances to reduce the pressure drop through a STHE.
Secondly, the baffles cause points of stagnation where the baffles intersect the inner surface of the casing. This causes inefficient heat transfer between the first and second fluid streams.